1 Hello friends and welcome to the tutorial on "Writing Python scripts"

     2 

     3 {{{ Show the slide containing title }}}

     4 

     5 {{{ Show the slide containing the outline slide }}}

     6 

     7 In this tutorial, we shall learn

     8 

     9  * How write Python scripts 

    10 

    11 Often we will have to reuse the code that we haave written. We do that by

    12 writing functions. Functions are bundled into packages and are imported as and

    13 required in the script.

    14 

    15 Let us first write a function that computes the gcd of two numbers and save it

    16 in a script.

    17 

    18 {{{ Open an editor and start typing out the following code }}}

    19 ::

    20 

    21     def gcd(a, b):

    22 

    23         while b:

    24             a, b = b, a%b

    25 

    26         return a

    27 

    28 We shall write an test function in the script that tests the gcd function every

    29 time the script is run.

    30 

    31 {{{ Add to the script }}}

    32 

    33 ::

    34 

    35     if gcd(40, 12) == 4:

    36         print "Everything OK"

    37     else:

    38         print "The GCD function is wrong"

    39 

    40 Let us save the file as script.py in /home/fossee/gcd_script.py

    41 

    42 We shall run the script by doing

    43 ::

    44 

    45     $ python /home/fossee/gcd_script.py

    46 

    47 We can see that the script is executed and everything is fine.

    48 

    49 What if we want to use the gcd function in some of our later scripts. This

    50 is also possible since every python file can be used as a module.

    51 

    52 But first, we shall understand what happens when you import a module.

    53 

    54 Open IPython and type

    55 ::

    56 

    57     import sys

    58     sys.path

    59 

    60 This is a list of locations where python searches for a module when it

    61 encounters an import statement.

    62 

    63 hence when we just did =import sys=, python searches for a file named sys.py or

    64 a folder named sys in all these locations one by one, until it finds one.

    65 

    66 We can place our script in any one of these locations and can import it.

    67 

    68 The first item in the list is an empty string which means the current working

    69 directory is also searched. 

    70 

    71 Alternatively, we can also import the module if we are working in same 

    72 directory where the script exists.

    73 

    74 Since we are in /home/fossee, we can simply do

    75 ::

    76 

    77     import gcd_script

    78     

    79 We can see that the gcd_script is imported. But the test code that we added at

    80 the end of the file is also executed.

    81 

    82 But we want the test code to be executed only when the file is run as a python 

    83 script and not when it is imported.

    84 

    85 This is possible by using =__name__= variable.

    86 

    87 First we shall look at how to use the idiom and then understand how it works.

    88 

    89 Go to the file and add

    90 ::

    91 

    92     if __name__ == "__main__":

    93         

    94 before the test code and indent the test code.

    95 

    96 Let us first run the code.

    97 ::

    98 

    99     $ python gcd_script.py

   100 

   101 We can see that the test runs successfully.

   102 

   103 Now we shall import the file

   104 ::

   105     

   106     import gcd_script

   107 

   108 We see that now the test code is not executed.

   109 

   110 The __name__ variable is local to every module and it is equal to __main__ only

   111 when the file is run as a script.

   112 

   113 hence all the code that goes after __name__ == "__main__" is executed only when

   114 the file is run as a python script.

   115 

   116 {{{ Show summary slide }}}

   117 

   118 This brings us to the end of the tutorial.

   119 we have learnt

   120 

   121  * What happens when we import a module

   122  * How to use a script as a module

   123  * How to write test functions using the __name__ idiom 

   124 

   125 {{{ Show the "sponsored by FOSSEE" slide }}}

   126 

   127 #[Nishanth]: Will add this line after all of us fix on one.

   128 This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India

   129 

   130 Hope you have enjoyed and found it useful.

   131 Thankyou

   132  

   133 .. Author              : Nishanth

   134    Internal Reviewer 1 : 

   135    Internal Reviewer 2 : 

   136    External Reviewer   :